1. Technical Field
The present invention relates to microwave circuits. More specifically, the present invention relates to multiplexers used to combine signals from two or more microwave channels.
While the present invention is described herein with reference to a particular embodiment in an illustrative application, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings of the present invention will recognize additional modifications and applications within the scope thereof.
2. Description of the Related Art
In microwave communication systems, it is not uncommon to transmit or receive several channels of voice or data through a single antenna feed. In such systems, each channel provides a separate communications link. It is highly desirable therefore to minimize cross coupling between the channels. To do so, many systems individually amplify and filter each channel prior to multiplexing the channels into the single feed via a waveguide multiplexer. Waveguide multiplexers usually consist of a common microwave waveguide (manifold) into which the several channels are slot coupled. (See FIG. 1.) For example, where filtering is desired prior to multiplexing, the channels are first input to a tuned cavity or resonant filter via a conventional coaxial line or slot (iris). Each filter is connected at its output end to a rectangular waveguide manifold via a slot in the broad wall, for example, resulting in a series connected multiplexer. FIGS. 1 and 2 illustrate this particular connection arrangement. Unfortunately, as shown in the radiation pattern of FIG. 3, the top wall slots strongly radiate and couple in the broadside direction. This forces a design constraint using the teachings of the related art. That is, the coupling of the slots in the broadside direction prevents two filters from being located in the same plane (one coupling through a slot in the top wall, while the other couples through a slot directly opposite in the bottom wall) as the mutual inteference therebetween would be maximum. Further, any slot represents a discontinuity which perturbs the fields, causing higher order modes. Two or more such discontinuities in close proximity can result in resonances and destructive interactions adversely affecting the performance of each filter. It is common practice therefore to separate, when possible, such discontinuities by a minimum of one quarter wavelength. This allows for a sufficient distance within which the higher order modes may attenuate. Thus, the next series connected node is typically one-half wavelength in distance down the manifold in accordance with the practice in the art of spacing multiplexer filters at half wavelength intervals.
Although the slot coupled designs have been used successfully for some time, the increasing demands of modern microwave communication systems has posed numerous problems. That is, modern systems require more and more communications channels. As the number of channels increases, however, the number of filters increases. Because of the need to space the filters, the increase in channels results in an increase in the length of the manifold. As the manifold is typically made of a conductor (eg. aluminum), an increase in length is accompanied by an increase in weight and associated cost. This is particularly true in regards to satellite communications systems.
Longer manifolds also create greater insertion losses, ie., those losses associated with the insertion of a component in a transmission line.
In addition to weight and insertion loss problems, those of skill in the art have observed that as the manifold lengthens, it becomes more susceptible to undersirable interfering resonances in the passband resulting from mutual coupling of the several slots.
Yet another problem results from the fact that the increased distance between filters causes the respective out-of-band impedances to become dispersed. Dispersion can result in performance degradation.
Longer manifolds are therefore more sensitive and difficult to tune. Finally, longer manifolds are more susceptible to performance degradations due to mechanical flexures.
It is generally desirable therefore to minimize the length of the multiplexer manifold.